Current transformer



p 1932- s. s. GREEN 1,875,590

CURRENT TRANSFORMER Filed June so. 1930 s Sheets-Sheet 1 ay zw ATTORNEYSept. 6, 1932. s. s. GREEN 1,875,590

CURRENT TRANSFORMER Filed June 30, 1930 3 Sheets-Sheet 2 a my J m 4 i AmJ M W M Mar/Z w w p 1932- s. s. GREEN 1,875,590

CURRENT TRANSFORMER Filed June 30. 1930 3 Sheets-Sheet 3 Patented Sept.6, 1932 UNITED STATES PATENT OFFICE STANLEY S. GREEN, OF LA FAYETTE,INDIANA, ASSIGNOR TO DUNCAN ELECTRIC MANU- FACTURING COMPANY, OF LAFAYETTE,

INDIANA, A CORPORATION OF ILLINOIS CU RREN T TRANSFORMER Applicationfiled June 80,

My invention relates generally to current transformers and has among itsobjects the production of such a transformer having great accuracy andefliciency, which is of economical construction and is light in weight.My invention is of particular utility in connection with currenttransformers of the single conductor or bar type but is not to be thuslimited.

In accordance with one characteristic of my invention, a laminatedmagnetic circuit devoidof breaks and gaps in the flux path is employedfor magnetically linking the primary and secondary transformer circuits.In accordance with another feature of the invention, secondary turns areafforded by providing separate secondary conductors which are passedthrough a winding space or aperture in the core. After these separateconductors are located in the desired position,

they are bent around the core to lap their contiguous ends and bringthem in contact throughout to electrically connect said secondaryconductors a substantial distance into a continuous secondary winding oflow resistance. This construction accomplishes the important result ofreducing the internal volt-ampere burden of the transformer andenhancing its accuracy. I desirably make the 80 winding aperture orspace small by working the primary and secondary conductors passingtherethrough at a high current density and providing heat-resistinginsulation for these conductors within such space so that the insulationwill not be damaged at full continuous rated current by the highconductor current densities involved. This winding space enables theemployment of a very short length of magnetic circuit which mosteffectively utilizes the core material and its inherent magneticcharacteristics. I desirably also dispose the magnetic core material toafford a very large area normal to the magnetic flux path. I alsopreferably make the core circular to give the shortest possible lengthto the flux path. An auxiliary reactance is preferably in the secondarycircuit and is adjusted to be relatively larger in its effect at lightcurrent loads and relatively less in its efl'ect at heavy current loads.Such aux- 1930. Serial No. 484,951.

iliary reactance secures a decrease in some of the errors of thetransformer, especially at light current loads.

The invention will be more fully explained n connectlon with theaccompanying drawings in which Fig. 1 is a perspective view of one formof current transformer embodying the invention; Fi 2 is a crosssectional view of the structure s own in Fig. 1, on a larger scale; Fig.3 is a side elevation; Fig. 4 is an axial sectional view; Fig. 5 is acurve diagram illustrating certain magnetic properties of a common formof magnetic material such as silicon steel at low magnetizing forces;Fig. 6 is a circuit diagram; Fig. 7 is a perspective view of one form oferror-compensatmg reactance; Fig. 8 illustrates a reactance core laminafor another form of reactance; Fig. 9 is a side elevation of atransformer similar to that of Fig. 3 embodying another form of errorcompensating reactance; and Fig. 10 is a perspective view of twosecondary conductors as assembled.

A bar type current transformer has only one primary stretch which passesdirectly through the core of the transformer. The total ampere turns onthe magnetic core of such a transformer is small as compared with thecore of a current transformer whose primary winding has many coils. Anerror producing component of the total primary ampere turns is involvedin the circulation of magnetic flux around the core, the greater suchcomponent the greater the error. Bar type current transformers, althoughsimple and economical, have always had as a disadvantage a tendency torather large errors both in ratio and phase angle due to the necessarilylarge component of the relatively small total primary ampere turns, thiscomponent 90 being consumed in order that the necessary magnetizingforce be supplied.

Hitherto current transformers have had built up cores containingsections or pieces enabling the location of the secondary windings onsuch cores and making the cores somewhat similar to the cores ofordinary current transformers having relatively long flux paths. Thispractice has resulted in relatively large errors.

In the form of transformer illustrated, ordinary current transformerpractice is departed from. The cause of the error,the small total numberof ampere turns on the core, is offset by so constructing the core as topermit successful operation with very small magnetizing force.

The transformer core 1 is built up of a large number of annular laminae1, preferably circular in contour and desirably having circular openingstogether constituting the bore 2 of the core. The bore or aperture ofthe core affords a winding space which is desirably coaxial with thecore. The individual annular laminze are continuous or endless to bedevoid of breaks in the direction of magnetic flux travel. Such breaksoccur in prior current transformers in order that the cores may be builtup and properly wound. "With good core construction and a large numberof ampere turns errors due to such breaks may be largely offset, but ina bar type transformer the error due to the breaks would be veryappreciable.

The only reluctance to the passage of flux furnished by the bar currentconductor in the transformer of my invention resides in the magneticmaterial of the laminae themselves. The shorter the flux path the higherthe available magneto motive force per unit of flux path length. Arelatively high magneto motive force per unit of fiux path length inmagnetic material produces more than a proportional correspondingincrease in the amount of flux along such path. The curve of this isillustrated in Fig. 5 which shows the permeability in relation tomagnetizing force, that is, magneto motive force per unit of flux pathlength, at low values of .such force for silicon steel, a commonly usedmagnetic material. Abscissae represent magnetizing force and ordinatesrepresent permeability. At very low magnetizing force values the initialpermeability may be very low, depending for its actual value uponinherent characteristics as well as heat treatment of the magnetic alloyitself. As the magnetizing force increases, the permeability alsoincreases and after some point as A on the curve is reached, thepermeability increases with extraordinary rapidity. In the currenttransformer illustrated the magnetizing force is made very high bygreatly shortening the magnetic path, whereby operation at all loads onthe transformer is secured as far out as possible towards .or evenbeyond such point. To this end the bore or winding space 2 is madepreferably circular and very small, the shortest flux path then beingthe immediate perimeter of such winding space. Other concentric fluxpaths about this aperture become progressively longer and hencerelatively less effective, but it is unnecessary to make the outsidediameter of the core very large if the internal diameter is very small,wherefore weight of magnetic material may be saved. In practice I havefound that an outside core diameter of more than three times theaperture diameter adds little to the performance, although this shouldnot be taken as a limitation. As I have designed my transformer all ironlies sufliciently close to the bore or winding space 2 to be worked mosteffectively with a relatively high magnetizing force.

In contrast to the undesirability of adding iron to the core byincreasing the outside diameter of the laminae punchings, it isdesirable and effective to add iron to the core by increasing the axiallength thereof to an extent or within the limit defined by desiredweight of the transformer, the dimensions desired as iron is added inthis manner being almost equally effective whether the core is long orshort. In practice I have found it desirable to employ an axial corelength that is more than twice the perimeter of the winding opening oraperture.

The aperture may be made very small by filling it with the primary andsecondary conductors and necessary insulation and working suchconductors at high current density at the rated full current of thetransformer. This permits the use of conductors of relatively smallcross-section and a minimum of insulation. In the embodiment of theinvention illustrated I arrange the secondary coils radially of the coreand evenly distribute them in a circle about a central primary conductor4 which is preferably cylindrical and coaxial with the core. Aninsulating sleeve 5 of heat resisting insulation snugly receives theprimary conductor. This sleeve is as thin as is consistent with theinsulating value and the desired mechanical strength. Another insulatingsleeve 6 snugly receives the inner sides of the secondary coils toinsulate them from the core 1. The secondary coils 3 are preferably madeapproximately rectangular in cross section and of such dimension thatthe inner sides of the required number, together with the necessaryinsulation on them will just fill the annular space between saidinsulating sleeves, which are coaxial with the core and each other. Theinsulation on the individual secondary coils 3 is preferably aheat-resisting enamel and the insulating sleeves 5 and 6 may be made ofsuch a material as bakelite with an asbestos base or of a micainsulating composition. The effectiveness of such construction may beindicated by comparing the core bore area or the bore perimeter with themaximum current which the transformer is capable of carryingcontinuously. For example a bore area of less than .002 square inchesand a bore perimeter of less than .012 per ampere of rated full currentof the transformer may then be employed. The compact arrangement ofinsulation and conductors within the core enables these results to besecured. v Because of the close proximity Suitably apertured end lates18 and 19 of the magnetic core which has a igh th'ermay be sllp ed overthe en s of primary inmal conductivity, heat is drained away from suchconductors and insulation very rapidly. The exam les shown are merelyfor illustration as t ere is really no definite minimum limit to thedimensions of the windin space except those that are defined by the carring point of the insulation within such space.

The core 1 may be of a high grade of silicon steel such as is ordinarilyused in current transformers. By annealing the core laminae aftercutting them to size, the initial permeability will be increased and thetransformer characteristics consequentl improved. Other core materialmay e employed such as steel having a relatively high nickel content andhigh initial permeability, or the core may be an intermixture oflaminacmade of such materials with some silicon steel laminae added, ifdesired.

The primary circuit including the one length of primary conductor 4 iseasily es tablished. To this end the circuit wires 7 and 8 may beattached to the flattened ends or lugs 9 and 10 of conductor 4 bysuitable bolts 11 and 12.

An insulating tube or sleeve 13 snugly receives the core. Insulating endrings 14 and 15 are received in the ends of sleeve 13 and lap theprimary insulating tube or sleeve 5. The secondary conductors may beinitially in straight lengths which are individually inserted in thecore bore. The end portions of each secondary conductor are bent acrossthe insulating end plates 14 and 15 and are laid across the outside ofinsulating tube or sleeve 13. Preferably the end portions of conductors3 on the outside surface of insulating tube 13 overlap approximately theoutside length of insulating tube 13. Adjacent consecutive overlappingend portions of adjacent conductors on the outside of tube 13 are thenelectrically joined as by solder at such points as 3 and 3 together withthe intermediate portion of conductor between such points if desired toform a complete secondary winding of the required number of turns. Theend turn terminals may be brought out at secondary terminal posts 16 and17. This method and construction not onlyv provides an easy means ofapplying secondary turns but actually secures a very low resistancesecondary winding. Eaci secondary turn has a small cross sectional areanormal to the current flow within the winding space 2 but a larger crosssectional area outside such space. Any decrease in secondary windingresistance reflects favorably on the accuracy of the transformer.

After the secondary conductors have been joined together to form acomplete winding, the outside surface thereof may be covered with a coatof insulating varnish and the entire exterior of the transformer coveredwith tape.

sulatin tn 5 and clamped together by throug bolts 20 to form an entiresolid assembly of the laminae windings and intervening parts. Footbrackets 20 may be su plied upon said end plates for mounting t etransformer. I

Because of the symmetrical ringlike construction of the core andsecondary winding as well as the small number of turns in such winding,the inherent or internal reactance of the secondary winding isrelatively low. There is a tendency to cause a rather lar e phase an leerror especially at light 10a s. This con ition may be im roved byproviding an artificial or an ad itional error compensating reactance 21for the secondary winding. Fig. 6 diagrammatically shows the secondaryof the current transformer serially connected with the current coil 3 ofa watthour meter, the reactance being shown in series with such coil.

Fig. 7 illustrates one form which reactance 21 may take. The reactancehere shown is inclusive of a number of annular laminae 21' havingaligned apertures 21 through which the reactance turns pass. Theselaminae may be constructed without breaks or joints so as to be endlessin the magnetic circuit and the apertures 21 may be as small aspossible. Such a reactance, being subjected to the entire magnetizingforce of the secondary current passing .through its turns, may be somade that its magnetic circuit becomes saturated at very light currentloads. That is at very light current loads the permeability of thereactance magnetic circuit will be high and its reactance will be highwhile at higher current loads, approaching full rated secondary current,the permeability will decrease and the reactance effect will decrease.Made in this way, the reactance may be made to exert an errorcompensating efi'ect predominating on light current values for thecurrent transformer where such compensating effect is most needed andfalling off in its effect as the load increases. The rate of reactancereduction, which is to say its rate of saturation, may be controlled byvarying the shape of the laminae.

Fig. 8 shows another form of reactance lamina having an aperture 21which has been punched ofl center so that a portion 21 will have a fluxconcentration as compared with other portions of the core and aconsequent early saturation. By varying the cross-sectional area of theflux path in the reactance core so that it is nonuniform at variouspoints, it is possible to get almost any desired rate of saturation inthe reactance with respect to the load current flowing. Fig. 9 shows analternate method of securing the auxiliary compensating reactance in thesecondary circuit of the transformer which consists in the insertion ofreactauce magnetic cores 22 on the outside of some of the secondaryturns. A number of such cores 22 are shown as tubes of magnetic materialwhich 5 have been slipped over the spliced secondary conductors 3 at theoutside of the insulating tube 13. The use of the error compensatingreactance with a. saturable core as the current increases is not limitedto type transformers,

although it is of particular service in such use effective therewith.

Changes may be made without departing from the invention.

Having thus described my invention, I

claim:

1. A current transformer including a closed continuous and unbrokenmagnetic core having a winding space, a primary conductor and asecondary winding, wherein the portions of the secondary winding on theoutside of such space have a greater efiective cross section normal tothe current flow than the portions within such space.

2. The method of building up a secondary winding on a tubular currenttransformer core, which consists in threading separate straight lengthsof conductor through the bore of the core, bending the ends of eachconductor length back upon the outside of the core, and thenelectrically connecting one end of a conductor length to the oppositeend of an adjacent conductor length on one side and its other end to theopposite end of an adjacent conductor length on its other side. 3. Themethod of building up a secondary winding on a tubular currenttransformer core which consists in threading separate straight lengthsof conductor through the bore of the core, bending the ends of eachconductor length back upon the outside of the core with the ends of eachconductor length in lapping contact with opposite ends respectively ofadjacent conductor lengths, and then electrically connecting theconductor lengths along their lapping portions.

4:. In a current transformer, a tubular core a primary formed of asingle stretch of conductor extending through the bore of the core, anda secondary winding formed of a plurality of separate conductor lengthseach threaded through the bore of the core, said conductor lengthshaving their ends bent back upon the outside of the core and thereoverlapped and soldered together along such overlap to form a continuouswinding.

5. The combination with a current transformer whose secondary windinghas a normal inherent self reactance, of an additional reactancecomprising an iron tube encompassing a wire of one of the secondarytransformer turns.

In witness whereof, I hereunto subscribe my name.

STANLEY S. GREEN.

